Project Summary. The primary auditory cortex (A1) is a central site of convergence for ascending sensory projections and projections from diverse neuromodulatory regions. These inputs interact in A1 to both influence moment-to-moment cortical activity and drive long-lasting changes in synaptic connections that may underlie auditory behavioral learning. Our recent work has revealed that a diverse group of inhibitory interneurons in cortical layer 1 (L1) has a privileged capacity to integrate tuned sensory input from the auditory thalamus and neuromodulatory inputs from cholinergic and serotonergic brain regions. These L1 interneurons send spatially- precise axonal projections to their postsynaptic targets that powerfully control cortical network activity and plasticity. Our recent results suggest that two distinct classes of L1 interneurons defined by the expression of either neuron-derived neurotrophic factor (NDNF) or vasoactive intestinal peptide (VIP) may receive differential inputs and send distinct outputs to their cortical targets. We propose that each L1 interneuron class has a specialized function in controlling cortical activity and plasticity. However, the mechanisms by which NDNF and VIP L1 interneuron subtypes are activated and control cortical state and plasticity are not fully understood. To address this unknown, we will use anatomical, trans-synaptic viral tracing, and electrophysiological approaches to elucidate the pre and post-synaptic partners of NDNF and VIP L1 interneurons in mouse A1. We will also use two-photon imaging in behaving mice to determine the in vivo activity and plasticity of NDNF and VIP L1 interneurons during auditory perceptual learning. The results of this research will identify L1 circuit mechanisms that promote auditory plasticity in adulthood and can be exploited to advance treatments following hearing loss.